Does Size Matter? Protons May Be Smaller Than Previously Thought

A new study, released this week in the journal Science, shows that even in the world of sub-atomic particles, size matters.

An international group of scientists led by Aldo Antognini, a physicist at the Max Planck Institute for Quantum Optics, say the proton, a basic building block of all matter, appears to be smaller than previously thought. This finding, which confirms a similar result the team showed in 2010, may open new questions about how sub-atomic particles react.

The proton is a fundamental constituent of the atomic nucleus, and one of the Universe’s most common particles. Unlike the electron or neutrino, which behave like points, the proton is a messy collection of quarks, gluons and virtual particles. This makes it difficult to measure with any accuracy. After three years of careful study, Nature.com reports, the reason for the discrepancy is no closer to being understood.

Ingo Sick, a physicist at the University of Basel in Switzerland told Nature’s Geoff Brumfiel, “Many people have tried, but none has been successful at elucidating the discrepancy.”

The team used laser spectroscopy to study the proton’s charge radius in a reconstituted hydrogen atom. The radius differs by about 4 percent from previous measurements, the study found.

In a telephone interview with Bloomberg, Antognini said that five research groups worldwide are working to determine why the different measurements exist and what they might mean. He explains that the result could cause a reassessment of certain keystone constants used to outline electromagnetic force controlling the actions of sub-atomic particles since the late 1940s.

“It´s important to have the proton right to push the science farther,” Antognini said.

For physics, hydrogen is a unique and important molecule. With just a single electron and photon, hydrogen is too small to hide anything, according to Antognini. The fields of quantum mechanics and electrodynamics have both emerged from physicists studying the interactions with hydrogen atoms.

The team explains the hydrogen atom’s emissions have a profound effect on physics. Hydrogen only emits or absorbs specific frequencies as electrons hop between orbitals, a fact that was critical in the development of quantum mechanics. Further research revealed that theses emission and absorption lines were actually closely spaced frequencies. This provided experimental validation of the Dirac equation, which led to the field of quantum eletrodynamics.

In the past, three methods have been used to measure protons. First, scientists measured protons by using electron scattering. This strategy shakes an electron beam into hydrogen gas, causing collisions between the electrons and protons. These collisions, along with the scatter, allow scientists to deduce proton radius. The second method used hydrogen spectroscopy to measure energy levels of electrons to deduce how large the proton might be.

Most recently, scientists have been using a particle called a muon, which is 200 times heavier than an electron, but carries many of the same properties. The team fired muons from a particle accelerator at the hydrogen atom, getting some of them to replace the electrons orbiting the proton. In 2010, by measuring the muonic hydrogen’s energy levels with a laser, they were able to extrapolate the proton’s size, which was about 4 percent — or 0.03 femtometers (fm) – smaller than deduced through previous methods.

This most recent study also used muonic hydrogen, but examined a different set of energy levels in the atom with a precision ten times greater than the first results. The new study confirmed the 2010 study’s findings – a proton radius of 0.84 fm. Antognini called the second result “totally compatible with the previous value.”

Even though the two studies using muonic hydrogen confirm each other’s results, they are incompatible with the older, non-muonic techniques. John Arrington, a nuclear physicist at Argonne National Laboratory, says errors in the muon-based measurements of the proton radius are unlikely to be to blame and yet it seems equally unlikely that all the other measurements are wrong too, according to Nature.

A difference of 4 percent seems negligible to the layman, but according to Antognini, it is significant to physicists. ArsTechnica reports that the original measurements put the size of the proton at 0.88fm, a difference of seven standard deviations from the new measurements. He compared it to a tiny crack in a dam; the amount of water seeping through the crack is “negligible compared to the water on the other side, but it may indicate a larger problem.”

The new measurement, if confirmed, may introduce new physics theories to explain the difference. This team is the only one to use muons to probe the proton — all of the other studies used electrons. The possibility, however small, exists that muons react differently from electrons. The effect would have to be tiny, or it would show up in other places such as the Large Hadron Collider. Confirmation is important as the finding may be due to some kind of an error, or it could be a basic problem with physics theories, as they currently exist.

“We want to be very, very cautious,” Antognini said.

Arrington and Sick both have their doubts. “I’m a big believer in our understanding of physics,” Arrington says. Given the power of existing theories, Sick says, the idea of fundamental differences between muons and electrons is “sort of hard to imagine”.

It is harder to imagine what has gone wrong. Everyone has gone back over their data more than once and mathematical equations have been recrunched. No problems have been identified, not even with the models used to estimate proton size from the measurements.